The present invention relates to an electric testing instrument and in particular and to an electric multi-purpose measuring instrument.
From German patent application No. DE-C1-33 02 509, a unipolar testing instrument having a flexible test prod and an LED display is known. This instrument can detect alternating voltages in mains and induced voltages in ungrounded conductors. The test prod of this conventional testing instrument is connected via a high-resistance protective resistor to the input of a Darlington amplifier circuit, the emitter of which is connected to the negative pole of a source of direct current, while the collector is connected via the LED and a current-limiting resistor to the positive pole of the voltage source. In this manner, even relatively weak induced voltages can be indicated by the lighting up of the LED, yet grounding of the testing instrument is not required. The input of the amplifier circuit is also connected via a resistor and a switching transistor to the negative pole of the voltage source. The base of the switching transistor is triggered by a square-wave generator, so that the switching transistor is periodically blocked and switched through. When the switching transistor is switched through, the amplification is decreased to such an extent that the LED lights up only when the full alternating voltage in the main is applied. Relatively weak ripple voltages are therefore indicated by a blinking LED display in the rhythm of the square-wave generator, while the a full alternating voltage in the main is indicated by a continuously illuminated LED. In this way the two types of voltage can be distinguished from one another without having to manually switch from one type of testing instrument sensitivity to another.
With the aid of the conventional testing instrument, alternating electromagnetic fields in the vicinity of voltage-carrying conductors can also be detected. This has the advantage that the test prod can be passed along the outside of the insulation on a conductor, and the voltage-carrying sections of the conductor's length can be identified. In this manner it is very simple to locate any breaks in an electrical line.
To detect electrical fields of this kind, the flexible test prod must be wound one or more times around the conductor, in order to assure sufficient coupling and so that a detectable voltage will be generated in the test prod. This has the advantage that it is relatively complicated to detect the alternating fields. Nor can this disadvantage be overcome, in the conventional testing instrument, by increasing its sensitivity, because in that case the amount of reduction in amplification engendered by the switching transistor cannot be regulated precisely enough to assure that a clear distinction between alternating voltage in the mains and induced voltages can be made.
From German patent application No. DE-C2-31 08 974, a similar testing instrument is also known, in which instead of the switching transistor, a contact electrode is provided. The contact electrode is grounded by the human body in order to reduce the amplification. This prior art, however, has the same disadvantage as the above-described testing instrument.
On the other hand, so-called multi-purpose measuring instruments having two test prods are known, which enable quantitative measurement of voltages, currents, and resistances in different sensitivity ranges. With such multi-purpose measuring instruments, it is presently impossible, as a rule, to detect breakage in voltage-carrying conductors by scanning the electrical fields in the vicinity of the conductor.